The Terps also secured wide receiver Marcus Fleming as a transfer from Nebraska.
The walk-on threw for 218 yards and two touchdowns against Rutgers.
The Terps were without Taulia Tagovailoa, Chance Campbell and Ayinde Eley, among others.
The redshirt sophomore transferred from Alabama this summer.
The Terps open their season Saturday with a road matchup against Northwestern.
Tagovailoa transferred from Alabama in May.
Jackson is one of four starters from last year's team to opt out.
Williams is the top-rated quarterback in the class of 2021.
The quarterback had announced his intent to leave the program in early February.
Lewandowski racked up 2,804 yards and 28 touchdowns in 10 games for Wisconsin-La Crosse last season.